My D800 (not E) raw files average around 40 MB each. According to a Google query, there are 1,048,576 MB in a Terabyte. So 1,048,576 / 40 = about 26,000 files.

That sounds low to me. Anyone care to correct my math?

RAID storage provides redundancy, so that a single failed drive doesn't necessarily cause data to be lost. It's designed for reliability and maximum uptime. But it's not the same as backup. Independent backup is most important.